The present invention relates to a cathode ray tube, particularly, to a cathode ray tube provided with an electron gun performing a dynamic astigmatic compensation.
In general, a color cathode ray tube comprises an envelope consisting of a panel P1 and a funnel P2 integrally fused to the panel P1, as shown in FIG. 1. A phosphor screen P3 (target) consisting of three phosphor layers emitting blue, green and red light rays, respectively, which are in the shape of stripes or dots, is formed on the inner surface of the panel P1. Also, a shadow mask P4 having a large number of apertures formed therethrough is mounted inside the phosphor screen P3 in a manner to face the phosphor screen P3. On the other hand, an electron gun P7 emitting three electron beams P6B, P6G, and P6R is arranged within a neck P5 of the funnel P2. The electron beams P6B, P6G, and P6R emitted from the electron gun P7 are deflected by horizontal and vertical deflection magnetic fields generated from a deflection yoke P8 mounted on the outside of the funnel P2. As a result, the phosphor screen P3 is scanned horizontally and vertically by these electron beams P6B, P6G, and P6R passing through the shadow mask P4 to strike the phosphor screen P4, thereby displaying a color picture image.
An in-line type color cathode ray tube of a self-convergence system is widely put to a practical use as a color cathode ray tube of the construction outlined above. In the in-line type color cathode ray tube, the electron gun P7 consists of an in-line type electron gun emitting the three electron beams P6B, P6G, and P6R aligned to form a row, i.e., a center beam P6G running on a single horizontal plane and a pair of side beams P6B and P6R running on both sides of the center beam P6G. The positions of the side beam holes in grids on the low voltage side and high voltage side of the main lens portion of the electron gun are deviated from each other to permit the three electron beams to be converged in the center of the screen. Also, the horizontal deflection magnetic field generated from the deflection yoke P8 is made to be of a pin cushion type, and a vertical deflection magnetic field generated from the deflection yoke P8 is made to be of a barrel type. By these particular constructions, the three electron beams P6B, P6G, P6R arranged to form a single row are self-converged on the entire region of the screen to provide the in-line type color cathode ray tube of a self-convergence system.
In the in-line type color cathode ray tube of the self-convergence system, the electron beam passing through a non-uniform magnetic field generally receives astigmatism and, thus, strains 11H and 11V are imparted to the electron beam as shown in FIG. 2A. As a result, the beam spot 12 of the electron beam in a periphery of the phosphor screen is distorted as shown in FIG. 2B. The deflecting distortion received by the electron beam, which is generated because the electron beam is put in an excessively focused state in the vertical direction, gives rise to a large halo (blurring) 13 in the vertical direction, as shown in FIG. 2B. The deflecting distortion received by the electron beam is increased with increase in the size of the tube and with increase in the deflecting angle so as to markedly deteriorate the resolution at the periphery of the phosphor screen.
Means for overcoming the deterioration of the resolution caused by the deflecting distortion is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 61-99249 and Japanese Patent Disclosure No. 2-72546. The electron gun disclosed in each of these prior arts is basically constructed as shown in FIG. 3. As shown in the drawing, the electron gun comprises first grid G1 to fifth grid G5. Also, an electron beam-generating section GE, a quadrupole lens QL, and a final focusing lens EL are formed in the order mentioned in the running direction of the electron beam. The quadrupole lens QL for each electron gun is formed by forming symmetrical electron beam holes 14a, 14b, 14c and 15a, 15b, 15c through the mutually facing surfaces of the adjacent electrode G3 and G4, as shown in FIGS. 4A and 4B, respectively. By allowing these quadrupole lens QL and the final focusing lens EL to be changed in synchronism with the change in the magnetic field generated from the deflection yoke, the electron beam deflected toward the periphery of the screen can be prevented from receiving the deflecting distortion of the deflecting magnetic field and, thus, from being markedly distorted. As a result, satisfactory beam spots can be obtained over the entire region of the screen.
The correcting means disclosed in the prior arts certainly makes it possible to eliminate the halo portion in the vertical direction of the electron beam spot. However, since a strong deflecting distortion is generated by the deflection yoke in the periphery of the screen, it is impossible to correct the phenomenon of the lateral deformation of the electron beam spot.
The problem inherent in the conventional electron gun will now be described with reference to FIG. 5 showing the lens operation of the conventional electron gun. Solid lines in FIG. 5 denote the orbit and lens function of the electron beam where the electron beam is focused on the center of the screen. Also, broken lines in FIG. 5 denote the orbit and lens function of the electron beam where the electron beam is focused in a periphery of the screen. In the conventional electron gun, a quadrupole lens QL is arranged on the side of the cathode of the main electron lens EL, as shown in FIG. 5. Where the electron beam is directed toward the center of the screen, the electron beam is focused on the screen by only the function of the main electron lens EL denoted by the solid line. On the other hand, if the electron beam is deflected toward a periphery of the screen, a deflecting lens DYL is generated by the deflecting magnetic field as denoted by the broken line in FIG. 5.
In general, a self-convergence type deflection magnetic field is utilized in a color cathode ray tube. Therefore, the focusing force is not changed in the horizontal direction (H), and a focusing lens as a deflection lens DYL is generated in only the vertical direction (V).
Incidentally, FIG. 5 is intended to point out the problem inherent in the self-convergence type deflecting magnetic field and, thus, the lens function of the deflecting magnetic field in the horizontal direction, i.e., within a horizontal plane, is not shown in the drawing.
When the deflecting lens DYL is generated, that is, when the electron beam is deflected toward a periphery of the screen, the electron lens EL is weakened as denoted by the broken line, and the quadrupole lens QL1 is generated to compensate for the focusing function in the horizontal direction (H), as denoted by the broken line. Also, the electron beam is allowed to run through the orbit denoted by the broken line so as to be focused on a periphery of the screen. When the electron beam is directed to the center of the screen, the principle plane of the lens for focusing the electron beam in the horizontal direction (H), i.e., within a horizontal plane (imaginary center of the lens, i.e., cross point between the orbit of the electron beam emitted from the electron gun and the orbit of the electron beam incident on the phosphor screen) is on a principle plane PPA. When the electron beam is deflected toward a periphery of the screen to generate a quadrupole lens, the principle plane in the horizontal direction (H) is moved to a principle plane PPB interposed between the main electron lens EL and the quadrupole lens QL1. Also, the position of the principle plane in the vertical direction (V) is moved from the principle plane PPA to the principle plane PPC. It follows that the position of the principle plane in the horizontal direction is moved backward from the principle plane PPA to the principle plane PPB, leading to a poor magnification. On the other hand, the principle plane PPA in the vertical direction is moved forward to the principle plane PPC so as to improve the magnification. As a result, a difference in magnification is generated between the horizontal direction and the vertical direction so as to elongate the electron beam spot in a lateral direction (lateral collapse or deformation phenomenon) in a periphery of the screen.
The present invention, which has been achieved in view of the problems described above, is intended to eliminate or moderate the lateral collapse phenomenon of the electron beam occurring in a periphery of the screen because of the difference in the lens magnification between the horizontal and vertical directions so as to obtain satisfactory image characteristics over the entire region of the screen.
According to one embodiment of the present invention, there is provided a cathode ray tube, comprising at least an electron gun including an electron beam forming section for forming and emitting at least one electron beam and a main electron lens portion for accelerating and focusing the electron beam, and a deflection yoke for generating a deflecting magnetic field for deflecting the electron beam emitted from the electron gun in horizontal and vertical directions on a screen to have the screen scanned by the deflected electron beam,
wherein:
the main electron lens portion consists of at least four electrodes arranged in the order of first, second, third and fourth grids;
an intermediate first voltage is applied to the first grid and an anode voltage is applied to the fourth grid;
the second and third grids that are positioned adjacent to each other are connected to each other via a resistor;
second and third voltages higher than the first voltage and lower than the anode voltage are applied to the second and third grids, respectively;
these first to fourth grids are arranged such that a second electrostatic capacitance between the second and third grids is smaller than any of a first electrostatic capacitance between the first and second grids and a third electrostatic capacitance between the third and fourth grids;
a first lens region is formed between the first and second grids;
a third lens region is formed between the third and fourth grids;
a second lens region is formed between the second and third grids; and
an asymmetric lens is formed in the second lens region.
In the cathode ray tube of the present invention, the electron beam has an electron lens system as shown in FIG. 12 and depicts an electron beam orbit under the lens function of the lens system. The solid lines in FIG. 12 denote the electron beam orbit and the lens function when the electron beam is focused in the center of the screen. Also, the broken lines denote the electron beam orbit and the lens function when the electron beam is focused on a periphery of the screen. As shown in FIG. 12, a quadrupole lens QL1 is formed in a central portion of a main electron lens EL in the electron gun of the present invention. When the electron beam is directed to the center of the screen, the quadrupole lens QL1 performs a diverging function in the horizontal direction and a focusing function in the vertical direction, as denoted by the solid lines. When the electron beam is deflected toward a periphery of the screen, the quadrupole lens QL1 performs a focusing function in the horizontal direction and a diverging function in the vertical direction, as denoted by the broken lines.
Since the quadrupole lens QL1 forms a diverging lens in the horizontal direction, i.e., within a horizontal plane, and a focusing lens in the vertical direction, i.e., within a vertical plane, when the electron beam is directed to the center of the screen, the main electron lens EL forms a substantially cylindrical lens having a strong focusing force in the horizontal direction so as to compensate for the difference in focus between the horizontal and vertical planes. If the electron beam is deflected toward a periphery of the screen, the main electron lens EL is weakened as a whole and is operated to cancel the lens function of the quadrupole lens QL1 in the horizontal direction.
In this case, the orbit in the vertical direction of the electron beam is as denoted by a broken line in FIG. 12. On the other hand, the orbit in the horizontal direction of the electron beam is as in the case where the electron beams are focused on the center of the screen because the position of the quadrupole lens and the position of the main electron lens are substantially coincident with each other.
Therefore, the principle plane of the lens (imaginary center of lens, i.e., cross point between the orbit of the beam emitted from the electron gun and the orbit of the beam incident on the screen) for focusing the electron beam in the horizontal direction (H) at the time when the electron beam is in the center of the screen is equal to that at the time when the electron beam is deflected toward the periphery of the screen (principle plane Axe2x80x2=principle plane Bxe2x80x2). In the vertical direction, the position of the principle plane is moved forward by the generation of a DY lens. In the conventional electron gun, the quadrupole lens QL1 is positioned closer to the cathode than the main electron lens EL. The electron beam is diverged by the quadrupole lens in the vertical direction, and the orbit of the electron beam extends through a point away from the axis of the main electron lens EL to cause the position C of the principle plane to be moved forward. In the electron gun of the present invention, however, the quadrupole lens QL1 is formed within the main electron lens EL. As a result, the orbit of the electron beam incident on the main electron lens EL remains unchanged and, thus, the principle plane Cxe2x80x2 in the vertical direction is moved to a position closer to the cathode than the principle plane C of the conventional electron gun. As a result, the magnification in the vertical direction is not larger than that in the conventional electron gun and, thus, the vertical diameter of the electron beam is not appreciably collapsed in the periphery of the screen.
Therefore, the amounts of deviation in the positions of the principle planes in the horizontal and vertical directions at the periphery of the screen are smaller in the electron gun of the present invention than in the conventional electron gun (magnifications in the vertical and horizontal directions are poor and satisfactory, respectively). As a result, the phenomenon of the lateral collapse or deformation of the electron beam at the periphery of the screen is suppressed to make it possible to obtain an electron beam having a substantially circular cross section.
As described above, the electron gun specified in the present invention makes it possible to obtain a cathode ray tube free from a lateral collapse of the electron beam at the periphery of the screen and exhibiting a satisfactory resolution over the entire region of the screen. Further, the second and third grids are connected to a resistor arranged in the vicinity of the electron gun. The anode voltage applied to the fourth grid is divided by the resistor, and the divided voltage is applied to these second and third grids, making it unnecessary to apply an extra voltage from outside the cathode ray tube. As a result, a cathode ray tube of a high quality as described above can be obtained easily.
It should also be noted that an AC voltage component is applied to the first grid. As a result, an AC voltage is overlapped with the DC voltage applied to each of the second and third grids via the electrostatic capacitance between adjacent electrodes. What should be noted is that a quadrupole lens is formed within the main lens between the second and third grids by the potential difference generated in this stage between the second and third grids.
Further, since the electrostatic capacitance between the second and third grids is smaller than any of the electrostatic capacitance between the first and second grids and the electrostatic capacitance between the third and fourth grids, the AC component generated by the AC component applied to the first grid and applied to the second grid is larger than that in the case where the electrostatic capacitance between the second and third grids is equal to or larger than any of the electrostatic capacitance between the first and second grids and the electrostatic capacitance between the third and fourth grids. Also, the AC component generated by the AC component applied to the first grid and applied to the third grid is diminished. Therefore, the potential difference between the second and third grids is increased. It follows that the AC voltage component applied to the first grid can be effectively utilized for formation and operation of the quadrupole lens formed between the second and third grids so as to diminish the AC component applied to the first grid.
Further, the second and third grids are connected to a resistor arranged in the vicinity of the electron gun. The anode voltage applied to the fourth grid is divided by the resistor, and the divided voltage is applied to these second and third grids, making it unnecessary to apply an extra voltage from outside the cathode ray tube. As a result, a cathode ray tube of a high quality as described above can be obtained easily.